Bisphosphonates such as 3-pyridyl-1-hydroxyethylidene-1,1-bisphosphonic acid (Risedronate) have been proposed for use in the treatment of diseases of bone and calcium metabolism. Such diseases include osteoporosis, hyperparathyroidism, hypercalcemia of malignancy, ostolytic bone metastases, myosistis ossifcans progressive, calcinoisis universalis, arthritis, neuritis, bursitis, tendonitis and other inflammatory conditions. Paget's disease and heterotropic ossification are currently successfully treated with both EHDP (ethane-1-hydroxy-1,1-diphosphonic acid) and Risedronate.
It is known in the literature that some bisphosphonic acids and their salts are capable of forming hydrates, Risedronate sodium exists in three hydration states: mono, hemipenta and anhydrous. Crystallization procedures which selectively yield the hemipentahydrate form over the exclusion of the monohydrate and anhydrous forms are desirable.